In a large-capacity power converter, the converter output is high voltage or large current, and therefore, such a large-capacity power converter is often configured with a plurality of converters multiplexed in series or parallel. Multiplexing converters can not only increase the converter capacity, but also reduce harmonics contained in an output voltage waveform by synthesizing outputs, and as a result, can reduce harmonic current flowing to a grid.
As means for multiplexing converters, there is a multilevel converter in which outputs of a plurality of converters are connected in cascade, and one of such multilevel converters is a modular multilevel converter. Each arm of the modular multilevel converter is composed of a plurality of converter cells connected in cascade.
Each of a first arm and a second arm for each phase of the conventional modular multilevel converter has a chopper cell (converter cell) and a reactor. The chopper cell has two semiconductor switches connected in series to each other, and a DC capacitor connected in parallel thereto. In each of the first arm and the second arm, the same number of chopper cells are connected in cascade via their respective output ends.
In the case of controlling each phase of the conventional modular multilevel converter, if the capacitor voltage of each DC capacitor cannot be kept constant, overvoltage or low voltage occurs on the capacitor voltage, resulting in a failure in which the device is stopped. Therefore, the following controls and the like are performed: average value control for causing the average value of voltage values of all DC capacitors for each phase to follow a capacitor voltage command value; individual balance control for causing the voltage value of each DC capacitor to follow the capacitor voltage command value; and arm balance control for causing the average value of voltage values of all the DC capacitors in the first arm and the average value of voltage values of all the DC capacitors in the second arm to coincide with each other.
Circulating current which circulates among the phases in the modular multilevel converter without flowing to the outside of the modular multilevel converter is controlled, a voltage command value is calculated so as to control AC current for each phase, and a DC voltage command value is calculated so as to control DC output terminal voltage (for example, see Patent Document 1 and Non-Patent Document 1 below).
In a conventional DC power transmission power conversion device, a plurality of power converters are DC-interconnected so as to perform transmission and reception of active power between different AC grids, and active power control and DC voltage control are performed for each power converter (hereinafter, may be referred to as an AC-DC conversion terminal), thereby adjusting AC active power. At this time, the minimum value in the active power control and the DC voltage control is selected, and one DC voltage command is set to be equal to or smaller than those for the other AC-DC conversion terminals.
As the characteristics of the AC-DC conversion terminal, when the DC voltage decreases, output of the DC voltage control increases and output of the active power control is selected. Thus, control is performed at the DC voltage level based on the AC-DC conversion terminal for which the voltage command is small, and the other AC-DC conversion terminals perform active power control operations. If AC grid failure occurs on the AC-DC conversion terminal for which voltage control is being performed, and power balance is lost, the DC voltage increases and the DC voltage control outputs of the other AC-DC conversion terminals decrease. Thus, the control switches to DC voltage control to continue the operation (for example, see Patent Document 2 below).